The invention relates to optical interferometers, such as the assignee's WYKO TOPO optical profiler, and more particularly to improvements that allow rapid, accurate determination of step heights between dissimilar materials having different phase change on reflection.
Although optical profilers have the advantage of being non-contact instruments, they have the drawback that they record errors in height variation across boundaries of different materials if the "optical phase change on reflection" varies across such boundaries.
An optical interferometric profiler encounters difficulties in accurately profiling surface height of a specularly reflecting surface having distinct areas of optically dissimilar materials. Optically dissimilar materials may be defined as materials (or material systems) that have a significantly different Fresnel phase change on reflection. Such phase changes on reflection lead to errors in the interferometrically determined height profile across boundaries of the dissimilar materials. More specifically, when light is reflected from a specularly reflecting test surface and interferes with light in a reference beam of an interferometer, the modulation or visibility of the interference fringes depends on the optical constants, e.g., the complex index of refraction, of the test surface materials. The interferometrically measured phase of the fringe pattern includes the phase change on reflection, which is a property of the optical constants of the reflecting material. In optical profilometry, optically dissimilar materials are those which produce a different phase change on reflection. For example, all dielectric (zero absorption) materials are optically similar, although they have different real refractive indices. The complex index of refraction is given by the term n.sub.c =n-ik, where n is the real part that indicates the amount of deflection of a ray of light passing through the material, and k is the imaginary part that indicates the amount of absorption.
The above-mentioned WYKO TOPO optical profiler marketed by the assignee readily measures or profiles surfaces of a sample material composed of a single material. The relative change in measured optical phase is related solely to the change in surface height of the sample, by the equation EQU .DELTA..PSI.=.DELTA.h(4.pi./.lambda.).
In the case of a sample composed of a single material, the above equation yields a very accurate measurement of surface height variation.
A problem occurs, however, when measuring samples with two or more dissimilar materials contained in the same field of view. While the variation in measured height is accurate within each area of a particular material, an error in measured height occurs across boundaries between dissimilar materials within the field of view due to the above-mentioned different phase change on reflection. The "measured" height change is given by the formula EQU .DELTA.h.sub.measured =.DELTA.h.sub.true +.epsilon.,
where .epsilon.=.DELTA..PHI..lambda./(4.pi.). In the above equation, .DELTA.h.sub.true is the actual height change and .DELTA..PHI. is equal to .PHI.1-.phi.2, .PHI.1 and .PHI.2 being the optical phase changes on reflection of the two dissimilar materials.
There is a presently unmet need for a technique to rapidly and accurately determine the height difference between areas of optically dissimilar materials within a field of view of an interferometer.